1. Technical Field
The present invention relates to acoustic sensors and methods for manufacturing the same. Specifically, the present invention relates to an MEMS (Micro Electro Mechanical Systems) type acoustic sensor, and a method for manufacturing the acoustic sensor using the MEMS technique.
2. Related Art
An electret capacitor microphone and an MEMS microphone are used for a miniature microphone, and embodiments of the present invention relate to an acoustic sensor (microphone chip) used in the MEMS microphone manufactured using the MEMS technique. First, the conventional acoustic sensor related to embodiments of the present invention will be described.
(General Acoustic Sensor MEMS Type of Prior Art)
FIG. 1A is a cross-sectional view in a diagonal direction of a conventional acoustic sensor 11 of the MEMS type (cross-sectional view taken along line X-X of FIG. 1B), and FIG. 1B is a plan view in a state a back plate is removed in the acoustic sensor 11. The acoustic sensor 11 is mainly configured by a silicon substrate 12 made of monocrystal silicon, a diaphragm 13 made of polysilicon, and a back plate 14. A back chamber 15 is opened in the silicon substrate 12 to pass therethrough, and the diaphragm 13 is arranged on an upper surface of the silicon substrate 12 so as to cover the upper side of the back chamber 15. The diaphragm 13 has a substantially square shape, where a beam portion 16 extends outward in the diagonal direction from each corner. Each beam portion 16 is fixed to the upper surface of the silicon substrate 12 by an anchor 17 arranged on the lower surface of the beam portion, and the diaphragm 13 floats from the upper surface of the silicon substrate 12 by the anchor 17.
The diaphragm 13 is covered by the back plate 14 fixed on the upper surface of the silicon substrate 12, and a great number of acoustic holes 18 (acoustic perforations) for passing the acoustic vibration is opened in the back plate 14. The back plate 14 includes a fixed electrode film 20 made of polysilicon on an inner surface of a plate portion 19 having high rigidity made of SiN.
The acoustic sensor 11 is manufactured through the manufacturing processes, for example, as shown in FIG. 2A to FIG. 2D (line appearing on the far side of the cross-section is sometimes omitted to facilitate the understanding in the cross-sectional views after FIGS. 2A to 2D) using the MEMS technique. As shown in FIG. 2A, a sacrifice layer 21 (SiO2 layer) is stacked on the upper surface of the silicon substrate 12, a polysilicon thin film is formed thereon, and the polysilicon thin film is etched to a predetermined diaphragm shape to form the diaphragm 13.
The sacrifice layer 21 (SiO2 layer) is further stacked on the diaphragm 12 and the sacrifice layer 21 to cover the diaphragm 13 with the sacrifice layer 21, and then the sacrifice layer 21 is etched in accordance with the inner surface shape of the back plate 14. A polysilicon layer is formed on the sacrifice layer 21, and such polysilicon layer is etched to a predetermined fixed electrode film shape to form the fixed electrode film 20. Thereafter, as shown in FIG. 2B, a SiN layer is stacked on the fixed electrode film 20 and the sacrifice layer 21, and the SiN layer is etched to a predetermined shape to form the plate portion 19. A great number of acoustic holes 18 are opened in the back plate 14 including the plate portion 19 and the fixed electrode film 20.
Subsequently, as shown in FIG. 2C, a central part of the silicon substrate 12 is selectively etched from the lower surface side to pass the back chamber 15 through the silicon substrate 12, so that the sacrifice layer 21 is exposed at the upper surface of the back chamber 15.
Thereafter, the sacrifice layer 21 is subjected to wet etching through the acoustic holes 18 of the back plate 14, the back chamber 15 of the silicon substrate 12, and the like, so that only the sacrifice layer 21 under the beam portion 16 remains as the anchor 17 and the other sacrifice layer 21 is removed, as shown in FIG. 2D. As a result, the diaphragm 13 floats from the upper surface of the silicon substrate 12 by the anchor 17 and is supported to be able to film vibrate on the back chamber 15, whereby an air gap is formed between the fixed electrode film 20 and the diaphragm 13.
In such acoustic sensor 11, the beam portion 16 is extended from the diaphragm 13 and the beam portion 16 is fixed to the silicon substrate 12 by the anchor 17 formed by leaving one part of the sacrifice layer to increase the displacement of the diaphragm 13 due to sound pressure and enhance the sensitivity of the diaphragm 13. In order to further enhance the sensitivity of the diaphragm 13, the beam portion 16 is made longer and the anchor 17 is positioned distant from the edge of the back chamber 15, and the length of the portion of the beam portion 16 not fixed with the anchor 17 is made longer.
However, the sacrifice layer 21 is etched with an etchant infiltrated from the acoustic holes 18 or an etchant introduced from the back chamber 15 as shown with arrows in FIG. 3A, and the anchor 17 is formed by the sacrifice layer 21 left under the beam portion 16 as shown in FIG. 3B. The anchor 17 thus also becomes long if the beam portion 16 is made long, and hence, the length of the portion of the beam portion 16 not fixed with the anchor 17 cannot be increased. Furthermore, the anchor 17 becomes thin and short if the etching time is extended in an attempt to position the anchor 17 distant from the edge of the back chamber 15 because the sacrifice layer 21 under the beam portion 16 also gets etched from the side surface as shown in FIG. 3C, and hence, the beam portion 16 cannot be supported with the anchor 17.
(Acoustic sensor of Japanese Unexamined Patent Publication No. 2009-89097)
The acoustic sensor in which the length of the portion of the beam portion not fixed with the anchor is increased is disclosed in Japanese Unexamined Patent Publication No. 2009-89097. In the acoustic sensor of Japanese Unexamined Patent Publication No. 2009-89097, the length of the portion of the beam portion not fixed with the anchor is increased by opening a plurality of through-holes at the portion of the beam portion not fixed with the anchor, and increasing the area at the end of the beam portion.
FIGS. 4B and 4C are a cross-sectional view and a plan view schematically showing the beam portion 16 formed with a plurality of through-holes 22 at the portion not fixed with the anchor 17. If the through-holes 22 are formed in the beam portion 16, the sacrifice layer 21 at the lower surface of the beam portion 16 will be etched by the etchant infiltrated from the through-holes 22, as shown in FIG. 4A when the sacrifice layer 21 is etched. Therefore, the sacrifice layer 21 remains only at the end where the through-hole 22 is not formed, and the anchor 17 is formed at the end of the beam portion 16, as shown in FIGS. 4B and 4C.
However, if the through-holes 22 are formed in the beam portion 16, the mechanical strength of the beam portion 16 may decrease, and the beam portion 16 may break during a drop test or when a device incorporating the acoustic sensor is dropped.
FIGS. 5A and 5B are a cross-sectional view and a plan view schematically showing the beam portion 16 in which the area at the end 23 is increased. According to such structure, the sacrifice layer remains at the end 23 even after the sacrifice layer is removed in a region other than the end of the beam portion 16, and hence, the anchor 17 can be formed at the end 23 of the diaphragm 13.
In such structure, however, the miniaturization of the acoustic sensor may be inhibited as the area of the beam portion 16 increases. Furthermore, the area of the anchor 17 becomes small compared to the area of the end 23 because the sacrifice layer other than the end 23 needs to be completely removed, and hence, the support of the diaphragm 13 may become unstable.